The optical luminosity LB is a parameter of
scale. Like the linear size, the range of median LB
values characteristic of classical
galaxies varies only slightly, until the latest types, where the
distinctiveness of the dwarfs becomes evident
(Figure 2b). The
ellipticals here are slightly brighter than spirals.

LUMINOSITY FUNCTION
Binggeli et al. (1988)
have carefully reviewed what is
currently known about the luminosity function (L). In their study of the
Virgo cluster
(Binggeli et al. 1985),
they derive the luminosity function
(L, T) for
each morphological type separately. The range of luminosities representative
of the classical galaxies is seen to be similar, as in
Figure 2b,
with the later spirals and dwarfs showing a characteristic decrease and being
clearly separate. Binggeli et al. find that while the brightest
galaxies are ellipticals, the most common galaxies are the dE's. Because
of morphological segregation, they conclude that (L, T) cannot be
universal. We discuss the effects of morphological segregation in
Section 4.

Holmberg's survey (1958)
included photometric parameters for both
magnitude and diameter with which he investigated the surface
brightnesses of galaxies and studied the effects of internal
extinction. Because internal extinction causes a sytematic change in
the observed surface brightness (and color) as a function of
inclination, we adopt a definition of surface magnitude that attempts
to account for extinction. As mentioned above, the issues of internal
extinction are still under significant debate
(Disney et al. 1989).

Figure 3a shows the global surface
brightness within D25,
B.
The distribution of B is nearly constant for the E, S0 and
classical spiral galaxies, but distinguishes clearly the late dwarf
categories Sm and Im. We emphasize again that,
as discussed by
Disney and Phillipps
(1983),
current catalogs are biased against systems of low surface brightness, and
some of the relative constancy across most of the classical
galaxy sequence may be due to catalog selection.

DISK SURFACE BRIGHTNESS AND SCALE LENGTH
By examining the available surface photometry,
Freeman (1970)
found that the face-on central surface brightnesses of most
spiral disks are nearly constant, with small scatter:
21.67 ± 0.30 B-mag arcsec-2. Deviations occur at the
ends of the spiral sequence among the S0's on the one side and
dwarfs on the other. Surprisingly, most E's also seem also
to have constant central surface brightness. With the caveat
that current catalogs are indeed biased against the low surface
brightness systems, most E, S0 and classical spiral
galaxies have the same scale length as a function of luminosity
regardless of morphology. A thorough discussion of the details
of this issue, including the effects of selection bias,
is presented in
Gilmore et al. (1990).